Thermally converted formulated liquid epoxide encapsulants, adhesives and coating compounds are extensively used within the electronics industry for bonding and encapsulating integrated circuit (IC), diode, and other microelectronic packages to printed circuit boards (PCBs). This is due to their unparalleled mechanical strength, adhesion properties and environmental stability, which combine to give excellent protection to electronic circuitry assembled on a PCB. Such liquid epoxide adhesives and encapsulants, also widely known in the industry as “glob-top” or chip-on-board (COB) encapsulants, have evolved into very sophisticated products used in very high production volumes. In one encapsulating procedure, a controlled volume of the encapsulant or adhesive is dispensed onto the top of a silicon IC (“die” or “chip”) located on the PCB. The encapsulant is then thermally converted to a fully crosslinked hard polymer network.
One major application area for this “glob-top” technique is in encapsulation of an IC or chip on a credit-card-sized plastic card. Following the encapsulation, the chip-carrying card is laminated with a similar sized card to form a credit card or other transaction card including the chip. The chip may be a microprocessor chip containing memory and a microprocessor. The chip can either be a microprocessor with internal memory or a memory chip with non-programmable logic. Connection to the chip is made either via direct physical contact, or remotely via a contactless electromagnetic interface. Such cards are often referred to in the electronics industry as “smart cards”.
In a typical procedure for encapsulating an IC chip to such a card, the chip is placed in position on the card and a predetermined volume (“glob”) of the premixed liquid epoxide encapsulant is accurately dispensed onto the chip. The volume of encapsulant is determined according to the dimensions of the chip to provide a layer of the encapsulant covering or encapsulating the chip. The card including the encapsulant-covered chip is then passed into an oven for thermal curing of the encapsulant to a fully crosslinked hard polymer network. A typical oven-curing operation for such an encapsulant or adhesive in the volume involved has a duration of between about two and eight hours depending on the type of encapsulant. Temperature rise and fall times during the curing operation must be carefully controlled. In many commercially available curing ovens it is necessary to maintain an inert gas atmosphere, such as a nitrogen atmosphere, in the oven. This is especially necessary for encapsulants in which an anhydride is included to promote curing.
The requirement for oven-curing of the liquid epoxide encapsulants adds considerably to the cost of manufacturing “smart-cards”. The ovens are expensive, occupy a relatively large volume of production space, for example, about 30 cubic meters (m3), and consume a relatively large amount of energy, for example, about 5 Kilowatts for an oven capable of curing 100 bonds per hour on cards having width of 75 millimeters (mm) and a length of 125 mm. Further, the form of the ovens makes them unsuitable for integration into an electronic assembly line. This requires that partly finished cards be taken from such an assembly line, processed as a batch in the curing oven, and then returned to the assembly line for completion. There is clearly a need for a liquid epoxide curing method that does not require such an oven-curing operation and that can be easily integrated into an electronic production line.